1) Good understanding of basic electronics
Diodes, Transistors - (NPN, PNP, NMOS, PMOS) TTL, CMOS logic family, OpAmps.
Reference Books: "Basic Electronics" by M.V. Rao
2) Basic knowledge of Digital Design
Boolean Algebra, Theorems, Combinational Circuits, Sequential Circuits, Finite State Machines
(FSM - Mealy & Moore)
Reference Books: "Fundamentals of Digital Circuits" by Anand Kumar
"Digital Design" by Morris Mano
3) Basic Knowledge on Hardware Description Language (HDL)
....like Verilog or VHDL & Basic knowledge of VLSI concepts & ASCI flow
Reference Books: "Verilog HDL" by Samir Palnitkar